The success of the Iron Dome missile shield over Israel last week in downing nearly 90% of incoming missiles its software deemed threatening is already being used as justification for more ambitious missile defenses.

“The fundamental fact is that this missile defense system saved the lives of many Israeli citizens and the live fire demonstration of this capability prevented the Israel military from going forward with an armed invasion of the Gaza Strip,” Riki Ellison of the independent Missile Defense Advocacy Alliance told his supporters. “In this time of increased proliferation of ballistic rockets and missiles by non-state actors and rouge nations and their use of them against innocent civilians, missile defense is absolutely fundamental, necessary, and needed.”

“Missile defense works,” hawkish military scholar Max Boot wrote on Commentary’s website midway through the conflict. “This is only the latest vindication for the vision of Ronald Reagan…”

It is certainly nothing short of amazing to view Israeli videos like this as incoming rockets from Gaza are blasted to smithereens overhead by Iron Dome’s Tamir interceptors.

But there are some key differences between what Israel faced last week – it is already at work on a more ambitious system — and the U.S. fledgling national missile defense system:

— Iron Dome was trying to down missiles with ranges of less than 50 miles. The U.S. national missile defense system is aimed at threats from across the Atlantic and Pacific oceans.— All of the Gaza projectiles apparently flew predictable arcs. “Steerable” missiles are just over the horizon.— Iron Dome was able to ignore two-thirds of the projectiles fired toward Israel because its battle-management software determined quickly that most of them were off course and posed scant threat to Israeli citizens. U.S. missile defenses won’t always have that option, depending on where in the incoming missile’s flight path the U.S. tries to shoot it down.— With some 1,500 fired over the course of a week, the Israel Defense Force was able to improve its targeting over time. Practice makes perfect, and no one is suggesting any foe of the U.S. will be ready to lob a barrage of hundreds of ICBMs toward it in the near-to-mid-term.— None of the Gaza rockets and missiles deployed decoys, which made every threatening missile a real target. A variety of decoys, some no doubt unanticipated, could bamboozle even the statest-of-the-art missile shield.

These characteristics wouldn’t be shared among any ocean-spanning missiles fired toward the United States by either Iran or North Korea. There’s another key difference, too: there’s great debate over whether either nation could produce a weaponized ICBM capable of reaching the U.S. — and would be willing to risk national suicide to do so.

So while Iron Dome’s success is laudable, it’s not – as they like to say at the Pentagon – scalable. Put simply, that means that simply building a bigger Iron Dome wouldn’t do for the U.S. what the original did last week for Israel.